The goals of the project are to elucidate the role of FoxP3+CD4+CD25+ natural Treg cells in HIV-1 replication and pathogenesis. The modulation of Treg cell levels in HIV-1 diseases is poorly understood. Even less is known about the role of Treg cells in HIV infection and pathogenesis. The confusion of Treg in HIV infection/pathogenesis is due to at least the following reasons: 1) Basic immuno-biology (development, phenotypes, and function) of Treg cells in human is poorly investigated due to lack of relevant models;2) Treg cells from blood as analyzed in most studies may not reflect the majority of Treg cells in lymphoid tissues;3) Treg cells may be affected differently at different HIV disease stages;and 4) individual difference in age and genetics within and between study populations. A robust animal model that addresses these limitations is urgently needed to study the modulation by, and role of Treg cells in, HIV infection. The DKO-hu HSC model is well-suited for this purpose. We can generate large "inbred" cohorts of DKO-hu mice with individual human fetal liver donor tissues (20-40 DKO-hu mice/donor FL tissue). With a stable functional human immune system, functional FoxP3+CD4+ Treg cells are developed in normal proportion in all lymphoid organs in DKO- hu mice. HIV-1 establishes persistent infection, with specific immune responses, immune hyperactivation and depletion of human CD4 T cells. I hypothesize that Treg cells are recruited or induced in lymphoid organs during acute HIV infection to modulate immune responses. These CD4+ Treg cells are also good HIV target cells that support high levels of HIV replication. Chronic HIV infection during AIDS progression or emergence of pathogenic HIV variants may impair/deplete functional Treg cells, leading to hyper-immune activation and AIDS progression. This project will investigate the role of FoxP3+ Treg cells in HIV-1 infection, replication and pathogenesis in vivo. First, we will study the development, function and maintenance of Treg cells in lymphoid tissues in vivo at different ages of DKO-hu mice, and their role in self tolerance. Second, we will study how HIV-1 infection affects number and function of FoxP3+ Treg cells in vivo. We will determine how HIV-1 infects and affects the proliferation and survival of Treg cells in lymphoid tissues. Third, I propose to test the role of Treg cells in early phase of infection and during chronic phased of HIV disease progression, by increasing or decreasing Treg cells in DKO-hu HSC mice before or during HIV-1 chronic infection. The proposal is based on our extensive preliminary results and our established experience in studying T cells and HIV pathogenesis in relevant models. Most importantly, the novel DKO-hu mouse has provided the first mouse model to directly investigate the function and role of human Treg cells during HIV infection in vivo. We will focus on the most fundamental questions of Treg cells in HIV infection and pathogenesis. Elucidation of the mechanism by which HIV-1 interacts with FoxP3+ Treg cells and their role in HIV-1 infection and AIDS pathogenesis will help us understand HIV-1 infection and pathogenesis. The findings will facilitate not only our understanding of Treg biology in HIV infection and pathogenesis, but also development of novel therapeutics. PUBLIC HEALTH RELEVANCE: We will investigate the most fundamental questions of how Treg cells modulate HIV infection and pathogenesis. Elucidation of the mechanism by which HIV-1 interacts with FoxP3+ Treg cells and their role in HIV-1 infection and AIDS pathogenesis will help us understand HIV-1 infection and pathogenesis. The findings will facilitate not only our understanding of Treg biology in HIV infection and pathogenesis, but also development of novel therapeutics.